Laser Cooling Enhances Atomic-Force Microscopy

CANBERRA, Australia, Aug. 19, 2014 — Cooling nanowire probes with lasers could lead to significant improvements in the sensitivity and resolution of atomic-force microscopes.

The technique, developed by a team from Australian National University, uses lasers to achieve a broadband multimode cooling of a nanowire probe of -23 dB to 8±1 K (about -265 °C).

This technique could make such microscope probes 20 times more sensitive and capable of detecting forces as small as the weight of an individual virus, the researchers said. They reported a room temperature force measurement sensitivity of <2 × 10-16N.

The researchers examine the gold-coated nanowire probe used in the study. Courtesy of the ANU Quantum Optics Group.
“The level of sensitivity achieved after cooling is accurate enough for us to sense the weight of a large virus that is 100 billion times lighter than a mosquito,” said Dr. Ben Buchler, a researcher with the ANU Research School of Physics and Engineering.

The force sensor used in the study was a 200-nm-wide silver gallium nanowire coated with gold.

“The laser makes the probe warp and move due to heat,” said doctoral candidate Giovanni Guccione, noting that the probes are also sensitive to vibration. “We have learned to control this warping effect and were able to use the effect to counter the thermal vibration of the probe.”

The probe cannot be used while the laser is on, the researchers said, as its effect can overwhelm the sensitive probe. So, with the laser off, measurements must be made within a few milliseconds before the probe heats up. By making measurements over a number of cycles of heating and cooling, the researchers said, an accurate value can be found.

“We now understand this cooling effect really well,” said doctoral candidate Harry Slatyer. “With clever data processing, we might be able to improve the sensitivity and even eliminate the need for a cooling laser.”
The researchers said their technique could benefit applications in biosensing, molecular metrology, subsurface imaging and accelerometry.